NEW YORK (Reuters) - Vertex Pharmaceuticals Inc. reported Wednesday the effectiveness of its experimental drug VX-950 for infection with hepatitis C virus (HCV) was as impressive in a trial involving hundreds of patients as it was in two smaller previous studies, according to an interim analysis.

"Although the latest results are preliminary, they are exciting and confirm findings from initial studies," said study leader Dr. John McHutchison, a Duke University professor.

The trial involves 250 patients infected HCV genotype 1, the most common but hardest-to-treat subtype. The treatment-na�ve patients received VX-950 for 12 weeks.

The U.S. study, one of three mid-stage trials either planned or already in progress, was designed to assess how well patients respond to the Vertex drug within 24 weeks after completing treatment. Patients were divided into four treatment groups.

Three groups received the Vertex drug for 12 weeks in combination with long-acting interferon and ribavirin. Another group received only interferon and ribavirin, along with a placebo.

Among 74 patients given VX-950 for whom data were available at the end of 12 weeks, 88% had undetectable levels of the virus. That compared with 52% of those in the group that received only interferon and ribavirin.

"In effect, almost 90% of patients had undetectable virus after the first 12 weeks of triple therapy in this trial," Dr. McHutchison said.

He said patients who became undetectable without 4 weeks of treatment, and continued to be undetectable at 12 weeks, will be allowed to stop taking all medicines, but will continue to be followed.

Others will take interferon and ribavirin alone for either an additional 12 or 36 weeks, but receive no additional VX-950.

"If the virus remains undetectable 24 weeks after treatment is stopped, you are considered to have cleared the virus," Dr. McHutchison said.

With interferon and ribavirin alone, only half of patients typically clear the virus after a full year of treatment. And the dual therapy can cause severe flu-like symptoms as well as depression.

Although the Vertex medicine by itself has been shown to wipe out more than 99.99% of virus in the bloodstream, it is not a stand-alone cure because the virus rebounds.

Vertex is hoping that by combining VX-950 with interferon and ribavirin -- but for far shorter periods than the two older drugs are commonly used -- it will deliver the final knockout punch.

Despite its apparent better potency, 9% of patients taking VX-950 dropped out of the latest study due to adverse events, including rash, gastrointestinal disorders and anemia -- compared with only 3% taking the standard dual therapy. Serious adverse events were seen in 3% of those taking the Vertex drug, compared with 1% taking just interferon and ribavirin.

While side effects were more pronounced among patients on VX-950, McHutchison said they should be weighed against the risks of continued hepatitis C infection.

He said the virus, formerly spread primarily by blood transfusions, but now more commonly contracted through shared needles or other exchange of body fluids, causes cirrhosis in about 20% of people infected for 20 years or longer. Its damage is the leading reason for liver transplants.